Piston with releasable rod connection for refrigeration compressors

ABSTRACT

A piston and connecting rod includes a connecting rod supporting a piston. The connecting rod has a piston end, with the piston being supported on the connecting rod so that it is moved away from the piston end during pivoting of the connecting rod in response to a predetermined angular movement of its crank end so as to maintain the compression ratio during the combustion phase and increase the dwell period of the piston prior to its descent. The piston includes a rounded recess underneath, and the connecting rod includes a terminal portion with a shape to pivot within the recess. An elastic element snaps into recesses of the piston to hold the rod and piston together.

The present invention refers to a particular kind of coupling used tolink a connecting rod with the associated piston.

To the purpose of more effectively exemplifying the present invention,said connecting rod is assumed to operate in close association with thecompression piston of a refrigeration compressor, in particular acompressor for household-type refrigerators, without this of courselimiting the scope of the same invention to just such a kind ofcoupling.

The practice is known from the patent EP 0 608 506 to WHIRLPOOL EUROPEB.V. of bringing about such a coupling between a piston of ahermetically sealed motor-driven compressor and the respectiveconnecting rod by means of a gudgeon pin that is adapted to engage,during the compression phase, the piston walls owing to mere pressureexerted against appropriate recesses provided in the same walls, whereasthe suction phase, which notoriously requires just a very small or evenno coupling force at all between connecting rod and gudgeon pin (infact, the pressure itself of the suction gas is such as to ensure thecoupling, again by pressure, between said elements), is carried out withthe connecting rod that keeps engaging the piston through an elasticelement, which has a substantially plane annular shape and is arrangedbetween said two elements.

Such a solution however, although representing a sure improvement withrespect to prior-art solutions from a general manufacturing point ofview, still has some drawbacks in that

the need still arises for two distinct component parts, ie. connectingrod and gudgeon pin, to be manufactured separately, whereas saidconnecting rod must anyway be provided with the ring required toaccomodate the gudgeon pin:

there still occurs a wear-down effect between said component parts, sothat there still is a need for a particular lubrication to be providedtherebetween:

the elastic element works by sliding, ie. rubbing against the gudgeonpin, so that it undergoes a particularly severe, rapid wear-down effect:

furthermore, the shape itself of the elastic element, due to its workingby bending under mere shear stress, does not ensure any absolutely firmcoupling between piston and gudgeon pin and this makes it possible forundesired noise to be generated.

From GB 2 235 961 a piston assembly for use with a hydraulic cylinder isknown; such a piston assembly is including a piston member defining arecess in an end face and an annular insert located in the recess forreceiving an end of a piston rod, the insert including first retentionmeans for retaining the piston rod therein, and second retention meanswhich co-operates with a surface of the piston member to inhibitwithdrawal of the insert from the recess.

However such form of realization requires complicated processing due tothe fact that the metallic piston has to be worked internally undercut.

Furthermore the use of resilient moulded plastic means 19 has to beavoided inside an hermetic compressor for refrigerators, as the combinedeffects of freon corrosion, lubricant oil corrosion and high temperaturewould rapidly destroy such a plastic material.

From U.S. Pat. No. 4,930,405 a compressor is shown having a connectingrod comprising a first and second eye, the first being connected bymeans of a wrist pin to the reciprocating piston, and the second eyebeing connected to a crank pin which is on one end of a crankshaft. Thiscrankshaft is orthogonal to the piston stroke and supportes at its otherend the rotor of an electric motor. The motor stator is supported on acrankcase that includes a crankshaft bearing portion, a cylinder, asuction and a discharge muffler. This crankcase is mounted within ahousing by means of coil springs and this housing is closed with the topportion forming a hermetically sealed unit.

The solution shown by said US patent comprises a two-piece connectingrod, and more specifically a new arrangement comprising a two-part rodthat permit easy mounting of the piston, the connecting rod and thecrank pin assembly without requiring the provision of additionalconnecting parts; however such a solution does maintain a traditionalmounting technique between the connectin rod and the piston as the wellknown wrist pin 15 is used for connecting the two parts, and thereforeno improvement in the connection between the connecting rod and thepiston can be found, and the relevant mass is unchanged.

The patent DE 3030319 shows a solution where the wrist pin is lodged inan hollow element connected with the internal surface of the pistonhead, instead of in proper holes on the cylindrical surface of saidpiston.

Such a form of realization does show some advantage from the assemblyoperation, but it implies a supplementary burdensome working to placeand fix such hollow element inside the piston.

It therefore would be desirable, and it actually is a main purpose ofthe present invention, to provide a technical solution that, whenapplied to the piston of a refrigeration compressor, eliminates theabove mentioned drawbacks and, in particular, simplifies bothconstruction and assembly, especially through a reduction in the partcount.

According to the present invention, such an aim is furthermore capableof being reached through the use of techniques and materials that aresimple. cost-effective and readily available on the market, without byany means impairing the performance capabilities and the reliability ofthe respective compressor.

These aims are reached in a type of coupling between connecting rod andpiston as described below by way of mere non-limiting example withreference to the accompanying drawings, in which:

FIGS. 1a, 1 b and 1 c are three perspective, partially cut-away views ofa piston, a respective connecting rod and a respective retainingelement, respectively, according to the present invention;

FIG. 2 is a perspective, partially cut-away view of the three componentparts shown in the three above cited Figures, after the assemblythereof.

With reference to the above listed Figures, it should first of all benoticed how the gudgeon pin, intended as a distinct mechanical elementhaving the function of linking the connecting rod with the piston, ispractically eliminated, while its function is carried out as follows:

the piston 1 is deprived of the holes in which the gudgeon pin isusually inserted, since, as pointed out above, said gudgeon pin issuppressed; on the contrary, the same piston 1 is provided, in its innersurface 2 opposing the working surface on which the pressure is exerted,with a recess 3 having the shape of a portion of a cylinder sectioned bya plane that is parallel to the axis a—a thereof. In other words, saidinner surface practically presents a dome in the shape of a portion of acylinder.

The connecting rod 4 presents in turn, in correspondence of its endportion facing the piston, a terminal portion 5 shaped as a portion of acylinder being so sized as to be able to couple, even only partially,with said recess 3 provided in the inner surface 2 of the piston.

It is anyway clear that, apart from the mutual compatibility of therelative dimensions, the minimum coupling ability requirement calls forthe radius of said two portions of cylinder 3 and 5 as described aboveto be substantially similar so as to enable the connecting rod to rotatethrough a limited arc with its terminal portion 5 within the recess 3,owing to the fact that the cylindrical surfaces facing each other at anymoment are always substantially in contact with each other.

Such a solution enables a twofold advantage to be obtained in terms of agreater common working surface between connecting rod and piston, with acorresponding reduction in the pressures involved and, therefore, alsoin the corresponding wear-down effect, and an enhanced lubricationeffect brought about by a substantially perfect coupling of the twocommon surfaces when working.

Furthermore, such a solution clearly makes it possible for even veryhigh stresses between connecting rod and piston to be transmitted duringthe compression phase, since these elements are in this phase undergoingpressure stresses that force them against each other, so that theproposed solution of common working surfaces gives full assurance of aneffective force transmission and, as a result, an effective,problem-free compression of the gas, with a low wear-down and anenhanced lubrication effect.

During the suction phase there is no particular stress or force thatmust be transmitted from the connecting rod to the piston, owing to thelow inertia of the latter and owing also to the fact that, in the firstpart of the suction stroke, the residual gas available in the cylinderusually keeps up a slight counter-pressure acting on the piston, so thatthe pulling force on the piston, already very low, is further reduced bysaid counter-pressure.

It is however necessary for the piston to remain linked with theconnecting rod also during the suction phase and, considering the lowforces required, it has been found that such a function can be carriedout in a fully satisfactory manner by such an elastic element 6 as bestillustrated in FIG. 1c which shows means in the shape of a closed loop,and having approximately the form of a saddle, in which two opposite“upper” portions 7 and 8 can be distinguished., along with two similarlyopposite “lower” portions 9 and 10, said upper and said lower elementsbeing arranged alternately.

Said upper portions 7 and 8 are adapted to engage, by simple contact andpressure, against respective reliefs 11 and 12 provided, as this is bestshown in FIG. 1b, on the lower side of the terminal portion 5 of saidconnecting rod, said reliefs being clearly not comprised in andpreferably opposite to the cylindrical surface in which the connectingrod terminates.

The “lower” portions 9 and 10 of said elastic element 6 are in turnadapted to engage respective recesses 13, 14 which, preferably in theform of horizontally oriented notches, are provided in appropriatepositions inside the inner cylindrical surface 15 of the piston, so asthis is best illustrated in FIG. 2.

In this manner, after completion of the assembly of the three elementsdescribed above, which can be carried out in different manners, and inparticular by taking advantage of the elasticity of the element 6, aconnecting rod-piston assembly is obtained which is surely effective foruse in the intended application, without any problem whatsoever arisingin the compression phase thanks to the afore described reasons, as wellas without any problem occurring in the suction phase since thesaddle-like configuration, or “arch”-like shape, of the elastic element6, along with the manner it engages the piston, makes it surely possiblefor the connecting rod to be also able to drive the piston without anyundesired lag or even the slightest delay, exactly for the reason thatthe arch-shaped structure of the elastic element 6; by working in acompressive manner during the suction phase, maintains its originalshape almost integrally.

In an advantageous manner, furthermore, the afore cited reliefs 11 and12 can have their surfaces contacting the upper portions 7 and 8 of theelastic element 6 reduced to mere segments, this being obtained bysharpening the lower extreme edges of said reliefs and giving saidcorresponding portions 7 and 8 the shape of a plane surface.

Furthermore, said sharp edges so formed by the lower borders of saidreliefs 11 and 12 must be arranged along the rotation axis b—b of theterminal portion 5 of said connecting rod with respect to the recess 3.Such a geometric arrangement, in fact, enables friction between saidconnecting rod and the elastic element 6 to be minimized, since theseelements only engage each other by pressure when working.

This solution enables the durability of the respective pressure contactsto be increased and, as a result, ebables the related lubricationrequirements to be simplified.

It will be appreciated that the above description and illustrations havebeen given, with reference to the accompanying drawings, to the merepurpose of exemplifying the present invention, and that a number ofvariants can be implemented and modifications can be made theretowithout departing from the scope of the present invention.

What is claimed is:
 1. Electric compressor, in particular for householdrefrigerating apparatuses, comprising: a hollow piston (1) without anythrough-hole provided for accomodating the gudgeon pin. a connecting rod(4), a recess (3) provided in the inner surface (2) of said piston onthe opposite side with respect to the working surface thereof, saidrecess being adapted to accomodate in a rotatable coupling form both theupper terminal portion of the connecting rod (4) and the couplingelements, one or more coupling elements (11, 12) adapted to link theupper terminal portion (5) of said connecting rod with said piston,elastic means (6) adapted to elastically lock said elements inside saidpiston and engage themselves, with respect to said piston, againstappropriate recesses (13, 14) provided in the inner cylindrical surface(15) thereof, said connecting rod and said coupling elements being madein a single-piece construction, and said coupling means being adapted toreciprocally engage said connecting rod and said piston, and said pistonbeing free from any through-passing holes provided to accomodate the endportions of said gudgeon pin, characterized in that said coupling meansare formed by two relief side extensions (11, 12) provided on said upperterminal portion (5) of the connecting rod, on opposite faces thereof.2. Electric compressor according to claim 1, characterized in that theinner contour of said recess (3) provided inside said piston and thecorresponding contour of the assembly formed by said upper terminalportion (5) and the respective coupling elements are semi-cylindricalcontours adapted to be coupled reciprocally.
 3. Electric compressoraccording to claim 2, characterized in that said elastic means (6) havean annular shape which is arcuated in a saddle-like manner toward thesmall end of the connecting rod so as to present, toward the zone whichis the closest to the small end of the connecting rod, two lower,preferably mtually opposing portions (9, 10).
 4. Electric compressoraccording to claim 3, characterized in that said portions (9, 10) areadapted to engage appropriate corresponding recesses (13, 14) that arepreferably made in a notch-like manner and are provided within the innercylindrical surface (15) of said piston.
 5. Electric compressoraccording to any of the preceding claims 2 to 4, characterized in thatsaid two relief extensions present a respective sharp edge facing saidelastic means, said sharp edges being aligned along the axis (b—b) ofrotation of said terminal portion (5) with respect to said piston, saidelastic means (6) being provided with substantially plane portions (7,8) oriented towards said relief extensions (11, 12) so that said sharpedges and said substantially plane portions (7, 8) are capable ofmutually engaging owing only to compression.